Abstract

We report on a molecular simulation study of the homogeneous nucleation of in the supercooled liquid at low pressure and for degrees of supercooling ranging from 32% to 60%. In all cases, regardless of the degree of supercooling, the structure of the crystal nuclei is that of the Pa3 phase, the thermodynamically stable phase. For the more moderate degree of supercooling of 32%, the nucleation is an activated process and requires a method to sample states of high free energy. In this work, we apply a series of bias potentials, which promote the ordering of the centers of mass of the molecules and allow us to gradually grow crystal nuclei. The reliability of the results so obtained is assessed by studying the evolution of the nuclei in the absence of any bias potential, and by determining their probability of growth. We estimate that the size of the critical nucleus, for which the probability of growth is 0.5, is molecules. Throughout the nucleation process, the crystal nuclei clearly exhibit a Pa3 structure, in apparent contradiction with Ostwald’s rule of stages. The other polymorphs have a much larger free energy. This makes their formation highly unlikely and accounts for the fact that the nucleation of proceeds directly in the stable Pa3 structure.

Received 24 January 2005Accepted 03 March 2005Published online 12 May 2005

Acknowledgments:

Financial support from Sanofi-Aventis is gratefully acknowledged. The authors also acknowledge the Centre Informatique National de l’Enseignement Supérieur for a substantial allocation of computer time.